Last week I gave Stewart Brand a simple challenge. In his book Whole Earth Discipline he claimed that the pesticide DDT "was banned worldwide" as a result of campaigning by environmentalists, killing millions. Complaints meant the explicit claim was cut at the last minute from the film he fronted for Channel 4, What the Green Movement Got Wrong, but the impression remained. I challenged Brand either to provide evidence to support his claim or to admit that he got it wrong.

None of them meet my challenge, but they have opened my eyes to what I fear is his real agenda – one that seems very different to what he claims to be doing. Here is the open letter I have just sent to him, in which I answer his new claims and voice my concerns.

Dear Stewart,

Thank you for your emails and sorry for taking a while to reply: I had to check your claims before responding to them. Before I go on, let me remind you of some of things you say in your book:

"Every interview with a public figure should include the question 'What have you been wrong about, and how did that change your views?' The answer will tell us if the person is intellectually honest or a tale-spinner with delusions of infallibility."

"Fessing up aids learning."

"Failure to acknowledge a mistake is paralysing."

Are we to assume that this advice applies only to other people? I have now given you several opportunities – before, during and after our televised debate - to admit that you made a mistake when you claimed, in relation to the control of malaria, that "DDT was banned worldwide". Instead of doing so, you have dug a deeper hole for yourself.

It's not a complicated matter. A worldwide ban cannot take place without an international instrument. The only instrument I'm aware of that regulates DDT at the global level is the 2001 Stockholm convention. This does not ban its use for disease control. Rather, DDT is banned for agricultural purposes, not least because spraying it indiscriminately encourages resistance in malarial mosquitoes. If there is another instrument, please name it and quote the relevant text. If there isn't, all you need to do is to admit that you got it wrong. As the passages I've quoted from your book suggest, there's no shame in doing so. The shame lies in sticking to a claim when it has been shown to be false.

Instead of answering my challenge, you begin your emails by selectively quoting two articles that make broad and generalised allegations (several of which turn out to be false), but which provide no evidence for a global ban. I took the trouble of reading these articles in full, and found that both of them, in sections you don't quote, make it clear that there was no global ban. That hardly helps your case.

Then you take the extraordinary step of turning for help to Patrick Moore. Moore, as I am sure you are aware, represents a number of polluting and destructive industries, and helps them with their battles against environmentalists. For example, he fronted the logging industry's campaign against people trying to stop the clear-cutting of old-growth forests in western Canada. He has been a prominent spokesman in the movement denying that man-made climate change is taking place, and has been criticised for failing to declare his commercial interests. He is the source of a number of lurid myths about the environment movement.

For example, in the Great Global Warming Swindle, broadcast on Channel 4 in 2007, he made the following claim: "When I left Greenpeace it was in the midst of them adopting a campaign to ban chlorine worldwide. Like I said, 'you guys, this is one of the elements in the periodic table, you know; I mean, I'm not sure if it's in our jurisdiction to be banning a whole element'."

But Greenpeace has never campaigned to ban chlorine worldwide. Instead it has campaigned against certain organo-chlorines, like dioxin, and sought – successfully – to stop chlorine being used to bleach wood pulp in Ontario and British Columbia because of the pollution it was causing in rivers and lakes.

This alone should make you wary of calling on Moore as an expert on what has and has not been banned worldwide. True to form, this master of spin tried to help you out by inventing a whole new concept in international law: a "de facto worldwide ban". What a marvellous weasel phrase that is - and how little it helps your case. Either there has been a worldwide ban or there hasn't. If there has, show me where and when.

It may be true, in some places and at some times, that DDT has been hard to obtain for the purposes of disease control when it was believed to be the most effective option, though you have yet to show me hard evidence that even this is the case. If it is true, however, I regret it. Like you, I believe that eliminating malaria by the most effective means has to be a global priority. In 1989 I got into trouble with a friend in Manaus in the Amazon whose house I was staying in by letting officials come in and spray his walls with DDT. I thought it was the right thing to do; he did not.

Moore also sent you an extract from his book in the hope that it might deliver you from the fix you're in. I checked out some of the claims he makes in this extract, and immediately found that they are exaggerated and inaccurate.

For example, he maintains that:

"DDT was even discontinued for use in malaria control by the World Health Organisation and USAID"

and that

"the WHO and USAID refused aid to countries that used DDT for malaria control"

These claims are untrue. Not only did these agencies not refuse aid on these grounds, they never stopped using DDT for malarial control, where they believed this was the best option. In many cases it was not the best option. As the malaria expert and former World Health Organisation (WHO) global malaria programme coordinator, Allan Schapira, points out in the Lancet (hat tip: Tim Lambert):

"Indoor residual spraying is an effective intervention, provided a programme infrastructure can be set up and maintained to include trained sprayers, supervisors, managers, stocks, equipment, and vehicles, that roads allow access to every village at the right time at least once a year, and that insecticides are not diverted to agriculture … In view of the difficulties encountered in maintaining indoor residual spraying, WHO has invested substantially in exploring other methods, especially insecticide-treated bed nets. … In the choice between indoor residual spraying and insecticide-treated nets, a WHO study group convened in 2004 noted that the decision should, in most cases, be based on operational factors. Because long-lasting insecticidal nets can be managed easily with minimum risk of diversion of insecticide, for most high-burden countries that have not developed an infrastructure for indoor residual spraying, the priority will be to ensure coverage of at-risk populations with such long-lasting nets."

"USAID has never had a 'policy' as such either 'for' or 'against' DDT for IRS [indoor residual spraying]. The real change in the past two years has been a new interest and emphasis on the use of IRS in general – with DDT or any other insecticide – as an effective malaria prevention strategy in tropical Africa."

That wasn't hard to find – it took me 20 seconds on Google – so why does Moore continue to spread this myth?

USAID also says that the use of DDT spraying "to prevent malaria is an allowable exception under the Stockholm convention" and that the decision about whether or not to use DDT "is based on cost-effectiveness; on entomological factors; on local building materials; and on host-country policy."

In other words, you have yet to show me any hard evidence of anything, let alone the claim that there was a worldwide ban. You can go on wasting my time and yours if you like, by seeking to defend something that is evidently false, or you could show a little of the intellectual honesty you call for in your book by accepting that you got it wrong. At the moment you are making yourself look like "a tale-spinner with delusions of infallibility", to use a phrase from your book.

In the meantime, all this has got me thinking: you call yourself an environmentalist and claim to be trying to reform the environmental movement for its own good. Yet you repeat and then try to defend a myth circulated by corporate lobbyists about the environment movement – that it obtained a global ban on DDT, killing millions – even when you are unable to produce evidence to support it. Then you turn for help to one of the environment movement's most entrenched enemies. Why should we continue to believe you when you claim that you're on our side?

Like Moore, you trade on your credentials as a founder of the early environment movement. Like Moore, you now work as a corporate consultant. By the way, who does your company, the Global Business Network, now represent? The list of corporations Sourcewatch gives as its clients – including ExxonMobil, Saudi Aramco, Cargill, Dow Chemical, Shell and BP – makes my hair stand on end. But is it correct? And why have you ceased to carry this list on your website? Like Moore, you attack the environment movement in ways that suit corporate interests: calling us, in effect, to drop our campaigns for regulation and democratic control in favour of techno-fixes.

When I first came across your work, I took it at face value. As I read more, I began to wonder if you are not, as you claim, pioneering a new form of environmentalism, but a new form of corporate consultancy. You seem to be seeking to shape the environmental debate to suit the businesses you work for. Our correspondence does nothing to dispel this impression. Can you disabuse me of my suspicions?

You are more dangerous than the other corporate-sponsored adversaries of the green movement. You don't deny that climate change is happening. You don't get abusive, you remain polite and charming, you sound reasonable at all times. You are, as a result, a more effective operator than them: you have persuaded a lot of influential people that you are working for the good of the planet. I fear you are running the most insidious and subtle exercise in corporate propaganda I have yet encountered. As a result, no one, until now, has called you out on it. With this response, that changes.